The automotive industry is keen to provide collision avoidance radar in automobiles for a number of applications, including autonomous intelligent cruise control systems (AICC), backup aids, rear approach warning systems, systems to facilitate the pre-crash operation of air bags, stop-go/urban cruise control systems and systems to facilitate pre-crash activation of air bags in side impact situations.
Radar systems that are technically capable of performing these applications are described in, for instance, U.S. Pat. No. 6,067,040 entitled “Low cost, high resolution radar for commercial and industrial applications” issued to K. V. Puglia on May 23, 2000, the contents of which are hereby incorporated by reference. These systems typically require two, identical short pulses to be generated. The first short pulse is transmitted and reflected from a target. The second short pulse is delayed by time equal to the round trip time from the radar transmitter and back. The second, delayed short pulse is used in the receive channel of the radar system as a gated local oscillator that is mixed with the returned, transmitted pulse. This results in a DC value indicative of the phase difference between the transmitted signal and the delayed signal. This phase difference can be analyzed to obtain the exact time delay and any Doppler shift of the return signal, giving both the range and the velocity of the target. In order to detect objects at different distances, it is necessary to be able to accurately vary the time delay of the second, delayed pulse so that it matches the time taken by the first pulse to go from the radar transmitter to the target and back. In that way the delayed pulse can be made to arrive at the mixer at the same time as the returned transmitted pulse for all possible target distances within the range of the radar.
Despite the title of the aforementioned patent, a common problem facing such radar systems is the high cost of implementation. A major reason for this high cost stems from their need for high resolution distance measurements (1–10 cm) over relatively short ranges (2–50 meters). This requirement translates into a need to generate very short pulses capable of being accurately delayed relative to a reference in time steps of approximately 125 pico-seconds. If such pulses are generated and controlled using conventional digital clocks and high speed counters, a raw clock speed of approximately 10 GHz is required. Such clocks are costly and complex to implement.